These are kind of expensive, but if you are going to be on the hook for a while or keep the boat for a while, the upgrade might be worth it.Perkins 4.108 Serpentine Belt Conversion Kit | Foley Engines
This was just a quick google, so I am sure there are cheaper kits out there.
I have heard horror stories about bent cranks on the 4 cyl moter because of over tightening. Do your research. I have twin 6.354 motors that I will be doing this upgrade on for the loop in about 4 years.
Good Luck

Had a 150 amp alternator on my 4-108, single belt.
Previous owner installed it in 1992. I sold the boat in 2013.
Never any problems, but the most charging I saw was 90 amps, had regulator obviously.
Alternator and regulator same brand, made in Texas, good stuff but forgot the name, Pro-line or some such thing.